This invention relates to polymer bound UV stabilizers. This invention also relates to the use of these polymer bound UV stabilizers to stabilize polymers or polymer blends against photochemical degradation.
When exposed to sunlight or to strong fluorescent illumination, most plastics undergo degradation. This usually results in color development and loss of physical properties. To overcome these problems, polymers are normally protected against photochemical attack by the incorporation of ultraviolet light stabilizers. UV stabilizers initially perform well in plastic compositions; however, they tend to be removed over a period of time by vaporization, migration or degradation under the action of elevated temperatures, or by the action of various solvents and cleaning agents, etc. The plastic compositions then degrade, discolor and in many instances become so brittle they can be easily broken.
Three factors affect the performance of the stabilizer in a polymer composition: the intrinsic activity of the stabilizer functional group on a molar basis, the compatibility or solubility of the stabilizer in the polymer system, and the ability of the stabilizer to remain in the polymer system. The third factor is often the dominant factor (G. Scott, New Developments in Rubber-Bound Antioxidants. Rubbercon 77, Int. Rubber Conf., 1977, 1, paper #19). Consequently, there has been considerable effort toward the development of stabilizers that are less volatile, more compatible and less readily lost during fabrication and exposure to the environment. Engineering thermoplastics are processed at high temperatures. A considerable amount of the additive may be lost when the hot polymers are exposed to the atmosphere or a vacuum (vented extruders) unless the additive has a very low vapor pressure. It is essential to use high molecular weight stabilizers that are not lost through drying, extrusion, and molding steps. Low volatility is also required for applications such as automotive paints where the stabilizer must suffer only minimal loss during oven drying and outdoor exposure (Kirk-Othmer Encyclopedia of Chemical Technology, third Edition, Vol 23, p 619, John Wiley & Sons, New York, NY). For polymers that come in contact with foodstuffs it is essential that the stabilizers be non-toxic or non-extractable from the polymer into the foodstuff. Many of the commercial low molecular weight UV stabilizers are too volatile to be used in these applications. Obviously, polymer bound stabilizers are preferred where FDA approval is required in the end-use.
Various approaches have been used to overcome volatility and compatibility shortcomings. One solution to the volatility and migration problems of UV stabilizers has been to prepare stabilizers with polymerizable groups. The choice then becomes to either polymerize the monomeric stabilizers to form homopolymers for use as additives, copolymerize the stabilizer with another monomer for use as an additive, or incorporate the stabilizer monomer in the preparation of the host polymer. In the homopolymer approach the monomers are often difficult to prepare and the homopolymers are usually incompatible with the polymers to be stabilized (J. Fertig, A. I. Goldberg and M. Shoultchi, J. Appl. Polym. Sci., 10, pp 663 (1966)).
For addition polymers and copolymers, the more popular approach is to copolymerize the polymerizable stabilizer with another monomer, thereby forming either a masterbatch for use as an additive, or simply a polymer or copolymer with sufficient stabilizer attached to the backbone of the (co)polymer chain. (D. bailey, O. Vogl, J. Macromol. Sci.--Rev. Macromol, Chem., C 14(2), pp 267-93 (1976)).
U.S. Pat. No. 4,042,773 demonstrates the attachment of UV absorbers to polymers and copolymers by first attaching the stabilizer onto the initiator used in the preparation of the polymer or copolymer to be stabilized.
Condensation polymers and copolymers have had stabilizer groups incorporated into the polymer backbones by reacting suitably functionalized UV stabilizers with reactive condensation monomers (U.S. Pat. Nos. 3,385,910, 3,391,110, 4,354,016, 3,862,087 and 3,213,058).
Monomeric stabilizers have been grafted onto polymeric backbones by melt processing in a Brabender Plasticizer. However, the extent of the grafting was quite low and most of the ungrafted stabilizer could be readily extracted out of the polymer blend (Y. N. Sharma, M. K. Naqvi, P. S. Gawande, I. S. Bhardwaj, J. Appl. Polym. Sci., 27, pp 2605-13 (1983)).
It is also known in the art to endcap polymers with reactive UV stabilizers (East German Patents 208,470 (CA101:193120z) and 208,471 (CA101:193114a)).
Another method of preparing polymer bound stabilizers is to attach stabilizer groups to existing polymers or copolymers. UV stabilizers containing thiol groups have been attached to rubber modified thermoplastics such as ABS or other rubber lattices using peroxide initiators (G. Scott, M. Ghaemy, Polym. Deg. and Stab., 3(1980-81), 253-263). European Patent Application 84,882 (CA99:141092n) discloses a method of attaching thiol UV stabilizers to rubber modified thermoplastics in the presence of a peroxide initiator in a melt processing step. U.S. Pat. No. 2,849,373 discloses the formation of an ionically bonded benzophenone stabilizer by reacting a polymer with pendant dimethylamino groups with 2-carboxy-2'-hydroxy-4'-methoxybenzophenone thereby producing a salt, with the UV stabilizer ionically bound to the polymer.
Considerable activity has involved modification of copolymers containing reactive functionalities with stabilizers containing groups that react with the reactive functionality of the copolymer. Two examples of this technique are the modification of glycidyl (meth)acrylate copolymers and the modification of maleic anhydride copolymers. Japanese Patent 71/26,860 (CA77: 20857c) covers the attachment of UV stabilizers containing amino groups, hydroxyl groups or isocyanate groups to crosslinked glycidyl methacrylate-divinylbenzene copolymers or crosslinked styrene-maleic anhydride-divinylbenzene copolymers. The attachment is through the respective polymer's reactive epoxide or anhydride groups. A particularly attractive approach to nonmigrating UV absorbers is demonstrated by the reaction of 2-hydroxy-4-(2-hydroxyethoxy)benzophenone with maleic anhydride grafted polyethylene to form a polymer bound 2-hydroxybenzophenone semi-ester (Japanese Patent 85/84,378, CA103: 161319w).